Abstract:

A single phase aqueous curable composition suitable for thermal inkjet
printing comprises a mixture of curable materials, water in amounts in
the range 10 to 30% of the weight of the compositions, and one or more
co-solvents for the curable materials. Because the water content is
relatively low, a drying step is not required before curing, increasing
printing speeds. The composition has good performance and has good
adhesion to a wide range of substrates including non-porous and
semi-porous materials as well as porous materials.

Claims:

1. A single phase aqueous curable composition suitable for thermal inkjet
printing, comprising a mixture of curable materials; water in an amount
in the range 10 to 30% of the weight of the composition; and one or more
co-solvents for the curable materials.

2. A composition according to claim 1, wherein water is present in an
amount in the range 10 to 25% by weight.

3. A composition according to claim 1, wherein water is present in an
amount in the range 15 to 20% by weight.

4. A composition according to claim 1, wherein the curable materials
comprise one or more acrylate monomers.

5. A composition according to claim 1, wherein the curable materials
comprise one or more trifunctional acrylate monomers.

6. A composition according to claim 5, comprising one or more alkoxylated
trimethylolpropane tri(meth)acrylates.

7. A composition according to claim 6, comprising one or more ethoxylated
trimethylolpropane triacrylates.

Description:

[0002]Inkjet printing processes fall into two main types: continuous
processes and drop-on-demand (DOD) processes. Continuous processes use
electrically conductive inks to produce a stream of drops electrically
charged ink that are deflected by an electric field to an appropriate
location on a substrate. In DOD processes, individual drops of ink are
expelled from the nozzle of a print head either by vibration of a
piezoelectric actuator (in piezoelectric inkjet printing) or by heating
the ink to form a vapourised gas bubble (in thermal inkjet printing, also
known as bubblejet printing). Thermal inkjet printing has advantages over
piezoelectric printing, with printers and print heads being lower cost
and with the printing process being able to achieve better resolution.

[0003]Inkjet inks need to satisfy a number of requirements, including the
following:

[0004]1. Viscosity must be appropriate. With DOD inks there are greater
limitations on inks for thermal printing than for piezoelectric printing,
with it generally being necessary for inks to have a viscosity of below 4
mPas at printhead operating temperature (typically 35 to 85° C.),
which usually equates to a viscosity of less than 12 mPas at room
temperature (25° C.), to be capable of being thermally inkjet
printed. In this specification, all viscosity values are at 25° C.
unless otherwise specified.

[0005]2. The ink must not cause unacceptable levels of clogging or
blockage of printing nozzles.

[0006]3. The ink must not result in build up of deposits on the ejection
heaters of thermal inkjet print heads (a process known as kogation) to an
unacceptable level during the working life of a printhead.

[0007]4. The ink should be stable in storage, without settling out or
coagulation of materials.

[0008]5. The resulting print needs to satisfy desired characteristics
depending on the field of use, with possible relevant factors including
water fastness, scratch resistance, durability, lack of shrinkage, lack
of cracking, flexibility, optical density (for coloured inks), uniformity
of deposition.

[0009]Conventional thermal inkjet printheads typically require use of an
ink with a high percentage (over 50% by weight) of water or volatile
organic solvent in order to achieve effective inkjet printing. While
generally a poor solvent for organic compounds and having too high a
surface tension to wet many plastic substrates, water may nevertheless be
the ideal solvent for coating and ink delivery, being able to lower
viscosity and volatilize without adding to emissions, toxicity or odour.
Aqueous inks, however, require proportionally longer drying times than
typical solvent-based inks, thus impacting on the maximum rate of
printing.

[0010]Increasing the print speed often means that the inkjet ink should
desirably dry more quickly such that the final print does not smudge when
stacked. Inks demonstrating reduced drying times can be achieved by, for
example, using volatile solvents in place of at least some of the water
in the liquid medium. However solvent-based inks are potentially
hazardous due to flammability issues and environmental concerns.

[0011]Inks with a quicker drying time due to lower water content, however,
often result in an increased tendency for the ink to dry in the nozzles
whilst the printer is not actively printing. Ink dried in nozzles tends
to cause difficulty in printing properly again after periods of rest.
Nozzles may become blocked, may fire intermittently or may fire
improperly, a kind of operability problem more specifically referred to
as decap.

[0012]Solvents such as alcohol or methyl ether ketone (MEK) are known to
improve (shorten) drying time, since such substances have higher vapour
pressure than water, and so evaporate faster. Also, these solvents have a
surface tension less than 25 dynes/cm, compared to 72 dynes/cm for water,
giving the solvents superior wetting characteristics on non-porous
substrates, and helping adhesion to the surface.

[0013]Various inks for inkjet printing processes are known in the art.
Generally, the inkjet inks used in the art are aqueous inks, comprising a
major quantity of water, a humectant and/or a co-solvent, together with a
colouring agent. By selecting specific surfactants, humectants, colouring
agents, or other components, it is possible to adjust the print
characteristics of the resultant ink.

[0014]Although numerous inkjet inks are presently available, they
generally do not meet all of the above-described requirements, while also
providing excellent print quality on the wide variety of plain papers
generally used in the home and office. Particularly, adhesion of these
inkjet inks to semi-porous and non-porous substrates is desirable.

[0015]Great effort has been expended in attempts to provide both dye-based
and pigment-based inkjet inks having acceptable adhesion on non-porous
substrates while maintaining other desirable characteristics. However,
there continues to be a demand for inks having all of the above-mentioned
desirable characteristics.

[0016]More recently, commercial interest in thermal inkjet inks has tended
to focus more on developing curable ink formulations, more specifically
radiation- or thermally-curable ink formulations, in which a UV (ultra
violet) or infra red lamp, or a combination thereof, is employed to bring
about a fast cure rate. The use of such devices necessarily reduces the
time-consuming process for removing solvent from the ink thereby
increasing the production speed, particularly in industrial printing
processes such as in mailing and addressing.

[0018]U.S. 2002/0198289 concerns UV-curable aqueous thermal inkjet inks
comprising UV-curable resin, suitable for printing waterfast images on
porous media. The examples all use a water content of over 35%, and there
is no teaching of how to make a low water content thermal inkjet ink.

[0019]U.S. Pat. No. 4,978,969 concerns UV-curable thermal inkjet inks with
good adhesion to plastics. The inks comprise UV-curable adhesive, and the
exemplified inks use a mixture of three solvents, water, methyl ethyl
ketone and gamma-butyrolactone, each at 5% by weight to ensure a
homogenous inkjet ink composition with viscosity and surface tension
properties suitable for use in inkjet printing. The benefits of organic
solvent-based inkjet inks are overshadowed by the environmental and
hazardous downsides of these components particularly the issues of
flammability and transportation. Moreover, inkjet ink compositions
comprising volatile liquids are often subject to an increased tendency
for the ink to dry in the nozzles. The document does not teach how to
make a single phase ink with a water content above 5% by weight.

[0020]WO 03/011989 describes a UV-curable ink for inkjet printing that is
free of aqueous and volatile organic solvents.

[0022]WO 2006/102524 discloses substantially non-aqueous (i.e. having a
water content of less than 10%) UV-curable thermal inkjet inks using a
volatile driver fluid such as acetone, methyl or ethyl acetate, methanol,
ethanol or propanol.

[0023]U.S. Pat. No. 6,790,875 discloses low viscosity curable thermal
inkjet inks, with the exemplified inks generally having a water content
in excess of 50% by weight.

[0025]Curable materials, e.g. monomers and oligomers, tend to have limited
solubility/miscibility in water, and substantial practical difficulties
arise in producing water-based inks with sufficiently low viscosity to be
useful for thermal inkjet printing that do not undergo undesirable phase
separation, do not cause clogging or blockage of printing nozzles, do not
result in unacceptable levels of kogation, and that produce prints with
appropriate, useful properties.

[0026]We have found that by use of mixtures of curable materials together
with a water-compatible solvent (referred to as co-solvent) for the
curable materials it is possible to produce low viscosity compositions
suitable for thermal inkjet printing having a reduced water content
compared to known aqueous thermal inkjet inks without needing to use
undesirable volatile organic solvents while still being capable of
achieving effective bubble nucleation.

[0028]In one aspect, the present invention provides a single phase aqueous
curable composition suitable for thermal inkjet printing, comprising a
mixture of curable materials; water in an amount in the range 10 to 30%
of the weight of the composition; and one or more co-solvents for the
curable materials.

[0029]The composition (also referred to as an ink) is suitable for thermal
inkjet printing and so generally has a viscosity of less than about 12
mPas, preferably less than about 11 mPas, more preferably less than about
6.5 mPas at 25° C. In this specification, all viscosities are at
25° C. unless otherwise stated.

[0030]The term "single phase" is used to mean that the curable materials
are fully in solution, being dissolved in water and co-solvent all being
fully missible therewith. No phase separation or settlement occurs. The
single phase composition may nevertheless possibly carry other materials
in solid form to achieve required application properties. For instance,
fine particles of pigment (which are insoluble in the composition) may be
carried in dispersion or suspension, often stabilised against
flocculation and settling by the use of dispersing agents, to provide
colour.

[0031]The curable materials harden on curing following exposure to a
suitable curing stimulus after printing to form printed material, usually
in the form of a film. The materials are typically radiation-curable,
curing in exponse to exposure to suitable radiation such as ultra violet
(UV) heat, infrared or electron beam radiation, with appropriate
initiators being used in the composition.

[0032]To produce useful compositions capable of being thermally inkjet
printed, the water content must be in excess of about 10% as otherwise
there is insufficient water for bubble formation and the inkjet cartridge
overheats.

[0033]By having a water content of less than about 30%, there is no need
for a drying step to remove excess water prior to curing. It is found
that inks with a water content in the range 10 to 30% can have good
jetting performance and produce films with good properties on porous,
semi-porous and non-porous substrates without the need for a drying step
thus enabling printing speeds to be increased.

[0034]The water content of the composition is preferably in the range 10
to 25%, 10 to 21%, yet more preferably 15 to 20%. The water is preferably
deionised (DI) water.

[0035]The curable materials are typically present in an amount in the
range 10 to 90% by weight of the composition, preferably 25 to 80%, more
preferably 50 to 70%.

[0036]The curable materials preferably comprise a mixture of different
curable monomers, typically including acrylate monomers. The different
curable materials are preferably chemically distinct, being from
different chemical classes, rather than being different grades of the
same material, e.g. having different chain lengths, molecular weights
etc.

[0037]The curable materials preferably include a mixture of acrylate
materials of different functionality, selected from monofunctional
acrylate, difunctional acrylate and trifunctional acrylate. Materials
with different functionality have different properties, e.g. in terms of
viscosity, water solubility and film-forming properties in terms of
hardness and water resistance, so by using a mixture of materials of
different functionality so a composition with a good overall balance of
properties can be achieved. The curable acrylate materials are typically
present in an amount in the range 40 to 50% by weight of the total weight
of the composition.

[0038]The curable materials preferably include one or more trifunctional
materials, desirably trifunctional acrylate monomers, suitably present in
an amount in the range 0.5 to 15% by weight of the composition,
preferably 2.5 to 12.5%, more preferably 5 to 10%, possibly in the range
1 to 10%, 1 to 9% or 3 to 9% . Trifunctional acrylates have the benefit
of being reactive, so increasing curing rate and cross-linking density,
and also having good solvent resistance, giving rise to good film
properties in terms of water/chemical resistance, scratch resistance and
durability. However, they also have high viscosities and produce films
with high shrinkage and low flexibility on curing so are generally
unsuitable for use without other curable materials.

[0039]The preferred class of trifunctional acrylate monomers is
alkoxylated trimethylolpropane tri(meth)acrylates. Materials in this
class have reasonable water solubility and produce printed films with
good properties, e.g. in terms of hardness and water resistance, but they
have relatively high viscosities. The materials in this class may be
ethoxylated (EO), propoxylated (PO) etc., with ethoxylated materials
generally being preferred as they have greater water solubility.
Alkoxylation helps water-miscibility of the material, and also the water
sensitivity of the resulting printed film. As the level of alkoxylation
increases, hydrophilicity increases with materials being more water
compatible and water soluble, with the consequence of producing more
water-sensitive prints. Ethoxylated materials are slightly more
hydrophilic than propoxylated materials, and ethoxylated materials are
generally preferred for this reason. Propoxylated materials function
acceptably, but need to be used at lower levels than ethoxylated
materials. As the level of alkoxylation increases, viscosity and
molecular weight also increase, and it is preferred to use materials
having no more than 20 ethylene oxide units (EO20), with lower levels
being favoured, typically no more than EO15, no more than EO9, no more
than EO6, with the preferred level being EO3. The currently preferred
material in this class is trimethylolpropane triacrylate with 3EO units
(EO3 TMPTA), having a good balance of properties including viscosity,
hydrophilicity and reactivity. This material has good water solubility,
fast cure response and low skin irritancy and produces printed films with
good properties including good water and solvent resistance and lack of
brittleness, i.e. good ductility.

[0040]Suitable commercially available UV-curable alkoxylated TMPTA
monomers include the following SR and CN materials available from
Sartomer, Miramer materials available from Rahn AG and Ebecryl materials
available from Cytec Industries (Sartomer, Miramer and Ebecryl are Trade
Marks):

[0041]EO3 TMPTA (SR 454, Miramer M3130, Ebecryl 160)

[0042]EO6 TMPTA (SR 499, Miramer M3160)

[0043]EO9 TMPTA (SR502, Miramer M3190)

[0044]EO15 TMPTA (SR9035, CN 435)

[0045]EO20 TMPTA (SR415)

[0046]PO3 TMPTA (SR492)

[0047]The curable materials preferably include one or more difunctional
materials, desirably difunctional acrylate monomers, suitably present in
an amount in the range 1 to 30% by weight of the composites, preferably 5
to 25%, more preferably 10 to 20%, possibly in the range 3 to 18% or 4 to
15%.

[0048]The difunctional acrylate preferably comprises one or more glycol
di(meth)acrylate, preferably polyalkene glycol di(meth)acrylates.
Diacrylates are generally preferred to dimethacrylates as they have
faster cure responses (which is advantageous) but higher viscosities
(which is disadvantageous). Compared with the triacrylates discussed
above, materials in this class have higher water solubility and lower
viscosity, but produce softer, more water sensitive films. The term
polyalkene is used to include propylene, dipropylene etc. The currently
preferred material in this class is dipropylene glycol diacrylate
(DPGDA), which has advantages of low viscosity, low volatility and fast
cure speed, and produces films with good flexibility, adhesion, hardness
and abrasion resistance.

[0049]Commercially available UV-curable difunctional acrylates include the
following:

[0073]The composition conveniently comprises a mixture of trifunctional
curable material and difunctional curable material at least, with the
difunctional material preferably being present in greater quantity than
the trifunctional material, suitably at about twice the amount.

[0074]The curable materials desirably include one or more monofunctional
materials, desirably monofunctional acrylate monomer, with such materials
having the benefit of low viscosity and good wetting properties. Suitable
monofunctional acrylate monomers include acrylates and methacrylates
etc., with acrylates being preferred. Monofunctional curable material is
preferably used in similar or equal amounts to difunctional curable
material, as discussed above, namely being present in an amount in the
range 1 to 30% by weight of the composition, preferably 5 to 25%, more
preferably 5 to 20%, yet more preferably 10 to 20%.

[0075]Commercially available UV-curable monofunctional acrylate monomer
materials include the following:

[0076]Tetrahydrofurfuryl acrylate (THFA) (SR285)

[0077]2-(2-ethoxy-ethoxy)ethyl acrylate (EOEOEA) (SR256, Miramer 170)

[0078]Tridecyl acrylate (SR489D)

[0079]Isodecyl acrylate (SR395)

[0080]2-phenoxyethyl acrylate (SR339, Ebecryl 114)

[0081]Caprolactone acrylate

[0082]Lauryl acrylate (SR335)

[0083]Tetrahydrofurfuryl methacrylate

[0084]Isobornyl acrylate (SR506)

[0085]Octodecyl acrylate (SR484)

[0086]Isooctyl acrylate (SR440)

[0087]Aliphatic monoacrylate blend (Ebecryl 113)

[0088]Alkoxylated THFA (CD611)

[0089]Cyclic trimethylolpropane formyl acetate (SR531)

[0090]The currently preferred monofunctional acrylate monomers are THFA
and EOEOEA.

[0091]The composition preferably includes a mixture of trifunctional
curable material and monofunctional curable material at least, most
preferably a mixture of trifunctional, difunctional and monofunctional
curable materials. By using a mixture of materials of different
functionality, a good overall balance of properties can be achieved. It
will be appreciated that different particular curable materials and
mixtures thereof may be better suited for different printing systems,
printing applications and/or printing media.

[0092]Materials having more than three functionalities can have benefits
of increasing cross-linking density, chemical resistance and scratch
resistance of printed films, but such materials have high viscosities and
so should be used, if present, in low quantities, typically not in excess
of 3%, e.g. in the range 1 to 2%. Good results have been obtained by use
of a small amount (about 1%) of a hyperbranched polyester acrylate
oligomer, particularly selected from those available from Sartomer under
the designations CN2300, CN2301 and CN3202, with CN2302 being preferred.
This material is a multifunctional dendrimer of generally spherical form
having about fifteen functionalities, thus providing very `fast surface
cure, yet comparatively low viscosity having regard to the number of
functionalities. These materials provide a combination of high
functionality with low shrinkage, and function well to improve film
toughness.

[0093]The composition may include one or more curable oligomers, typically
in small amounts e.g. in the range 0.1 to 2% by weight of the weight of
the composition. Oligomers may be used to adjust various physical
properties of the cured printed film, including chemical resistance,
flexibility, weatherability, and shrinkage. Suitable oligomers include,
but are not limited to: polyesters and acrylics. The CN2300, CN2301 and
CN2302 oligomers referred to above are suitable for use under this
heading.

[0094]The composition desirably further includes one or more curable
n-vinyl monomer materials, preferably a mixture of two or more n-vinyl
monomer materials. Such materials generally have low viscosities and so
act to reduce the viscosity of the composition. The materials also act as
curable humectants, helping jetting stability and printing reliability
and preventing drying on the nozzles because they are non-volatile. They
cure in with the other curable materials, increasing film thickness.
Because they cure in with the other materials they do not have to be
removed from the print before or after curing. These materials are also
found to have beneficial anti-kogation properties. In addition, they
function as co-solvents, to be discussed below.

[0095]Preferred n-vinyl materials include n-acryloyl morpholine (NAM),
n-methyl-n-vinyl acetamide (NMNVAM) and n-vinyl acetamide (NVAM), with it
being preferred to use a mixture of NAM together with NNMVAM and/or NVAM.
n-vinyl pyrolydone (NVP) is also useful but it is preferred to avoid this
material for health and safety reasons.

[0096]The n-vinyl monomer materials are typically used in an amount of up
to about 50% by weight of the total weight of the composition, typically
at levels of 45%, 40%, 35% or less, eg. in the range 10 to 50%,
preferably 15 to 45% more preferably 20 to 42%. NAM is conveniently
present in an amount in the range 5 to 25%, preferably 10 to 25%, more
preferably 15 to 22%, and NMNVAM/NVAM is conveniently present in an
amount in the range 1 to 25%, preferably 4 to 20%.

[0099]It is preferred not to use low boiling point solvents, having a
boiling point below 80° C., except in low amounts, less than about
5%, preferably less than about 3% because of flammability risks, although
such materials can impart useful properties. They have very low surface
tension so act as wetting agents and give good image quality on certain
substrates such as polyethylene, polypropylene, ABS.

[0100]Non-curable co-solvent is preferably present in an amount of less
than 15% by weight of the composition, more preferably less than 10%, as
such mterial is likely to remain in the printed film and could adversely
affect film properties. The total amount of co-solvent (curable or
otherwise) is typically in the range 20 to 45% by weight of the total
weight of the composition, preferably 25 to 40%, more preferably 30 to
40%.

[0101]In one particular embodiment of the invention, the composition
includes a co-solvent which does not also function as a monomer material.
Such a co-solvent may be used in conjunction with a curable monomer
material.

[0102]In another particular embodiment of the invention, the comparison
includes only co-solvent which also functions as a monomer material. Of
particular interest are compositions using one or more of n-acryloyl
morpholine, n-vinyl acetamide and n-methyl-n-vinyl acetamide as monomer
and co-solvent, possibly without other non-curable co-solvent materials.

[0103]In a preferred aspect, the invention provides a single phase aqueous
radiation-curable ink composition with reduced aqueous solvent content
having a viscosity of less than 11 mPa s, more preferably less than 6.5
mPas at room temperature (25° C.) and suitable for thermal inkjet
printing, comprising a mixture of curable materials including at least
three curable materials, the curable materials including a monofunctional
acrylate, an alkoxylated trimethylolpropane acrylate and a polyalkene
glycol acrylate; and one or more co-solvents for the curable materials.

[0104]A further aspect of the invention resides in a single phase aqueous
radiation-curable ink composition with reduced aqueous solvent content
having a viscosity of less than 11 mPas, more preferably less than 6.5
mPas at room temperature (25° C.) and suitable for thermal inkjet
printing, comprising a mixture of curable materials including at least
three curable materials, the curable materials including a monofunctional
acrylate, an alkoxylated trimethylolpropane acrylate, a polyalkene glycol
acrylate, and one or more n-vinyl compounds; and one or more co-solvents
for the curable materials.

[0106]In another aspect, the invention provides a single phase aqueous
radiation-curable ink composition with reduced aqueous solvent content
having a viscosity of less than 11 mPas, more preferably less than 6.5
mPas at room temperature (25° C.) and suitable for thermal inkjet
printing, comprising a mixture of curable materials including at least
two curable materials and at least two curable n-vinyl monomers.

[0107]Mixtures of materials may be used as the co-solvent. One preferred
co-solvent comprises a mixture of n-acryloyl morpholine (which is
curable, has a high boiling point and reasonable viscosity) and ethylene
glycol monobutyl ether (EGMBE). The n-acryloyl morpholine is conveniently
present in an amount of about 20% of the weight of the composition and
the EGMBE comprises about 6% of the weight composition. A further
preferred co-solvent comprises a mixture of 1,3
dimethyl-2-imidazolidinone (in an amount of about 10% of the weight of
the composition) and n-acryloyl morpholine (in an amount of about 20% of
the weight of the composition).

[0108]Viscosity values for various radiation-curable materials are given
below:

[0109]A further aspect of the invention provides a single phase aqueous
radiation-curable ink composition with reduced aqueous solvent content
having a viscosity of less than 11 mPas, more preferably less than 6.5
mPas at room temperature (25° C.) and suitable for thermal inkjet
printing, comprising one or more curable materials; one or more
co-solvents for the curable materials; and an anti-kogation agent
selected from n-vinyl acetamide (NVAM), n-methyl-n-vinyl acetamide
(NM-NVAM), and mixtures thereof.

[0110]Good results have been obtained with an anti-kogation agent
comprising a mixture of n-vinyl acetamide and n-methyl-n-vinyl acetamide.

[0111]The composition optionally includes surfactant in small amount (say
up to 3% by weight) to improve wetting. Suitable surfactants can be
selected having regard to the substrate on which the composition is to be
printed. Suitable surfactants include the following:

[0112]Surfactants e.g. Tego Glide 410 Wet 425, Tego 510, e.g. in amounts
of about 0.1% by weight, and Tego Protect 5000 and Tego Protect 5100 in
amounts of 2.8 to 3.0% by weight have been found to give good results.

[0113]Coating and ink additives as anti-kogation agents e.g. Tego Protect
5000 and Tego Protect 5100, e.g. in amounts of 2 to 3% by weight, are
currently favoured.

[0114]The composition optionally includes coating and ink additives for
use as anti-kogation agents in small amount (say up to 3% by weight) to
prevent drying in the printhead nozzles. These anti-kogation agents
ideally have good compatibility with water, low viscosity and good
anti-puddling properties. Suitable coating and ink additives for use as
anti-kogation agents include, but are not limited to the following from
Degussa AG: Tego Flow ATF 2 (anti-crater and flow additive), the slip and
flow additives (Tego Glide A115, Tego Glide 410, Tego Glide 432, Tego
Glide 435, Tego Glide 440 and Tego Glide 482), the release additives
(Tego Rad 2600 and Tego Rad 2700), the solvent-based binders for
production of release coatings (Silikoftal non-stick 60 and Silikophen
300), the linear organofunctional polysiloxanes (Tegomer A-Si 2322,
Tegomer C--Si 2142 and Tegomer C--Si 2342), (Tegomer D-3123 (polyether
diol), the modified poly-dimethylsiloxane resins for the production of
solvent-based anti-graffiti coatings (Tego Protect 5000 and Tego Protect
5100), and the silicone-polyacrylate resins for the production of
solvent-based anti-graffiti coatings (Tego Protect 5001 and Tego Protect
5002). (Tego, Silikophen, Silikoftal and Tegomer are Trade Marks).

[0115]The term "radiation-curable" refers to functionality directly or
indirectly pendant from a surface-treated particle, monomer, oligomer,
polymer, or other constituent (as the case may be) that participate in
polymerization and/or crosslinking reactions upon exposure to a suitable
source of curing energy. Such functionality generally includes not only
groups that cure via a cationic mechanism upon energy exposure but also
groups that cure via a free-radical mechanism. Representative examples of
radiation-curable groups suitable in the practice of the present
invention include epoxy groups, (meth)acrylate groups, olefinic
carbon-carbon double bonds, allyloxy groups, alpha-methyl styrene groups,
(meth)acrylamide groups, cyanate ester groups, vinyl ethers groups,
combinations of these, and the like. Free-radically polymerizable groups
are preferred. Of these, (meth)acryl moieties are most preferred. The
term "(meth)acryl", as used herein, encompasses acryl and/or methacryl.

[0116]The energy source used for achieving polymerization and/or
crosslinking of the curable functionality may be actinic (e.g. radiation
having a wavelength in the ultraviolet or visible region of the
spectrum), accelerated particles (e.g. electron beam radiation), thermal
(e.g. heat or infrared radiation), or the like. Preferably, the energy is
actinic radiation, because such energy provides excellent control over
the initiation and rate of polymerization and/or crosslinking.
Additionally, actinic radiation can be used for curing at relatively low
temperatures. This avoids degrading or evaporating components that might
be sensitive to the relatively high temperatures that might be required
to initiate polymerization and/or crosslinking of the energy curable
groups when using thermal curing techniques. Suitable sources of curing
energy include lasers, electron beams, mercury lamps, xenon lamps, carbon
arc lamps, tungsten filament lamps, sunlight, low intensity ultraviolet
light (UV), and the like. The use of UV light for polymerization tends to
form higher molecular weight polymers as compared to many other kinds of
curing energy. Accordingly, when it is desired to form higher molecular
weight materials upon curing, the use of UV light is preferred.

[0117]The ink composition typically includes a suitable initiator, either
alone or in combination of two or more, appropriate to the curable
materials, e.g. a thermal initiator, photoinitiator etc. Suitable
initiators are well known to those skilled in the art, as are suitable
levels of use (typically less than about 6% by weight).

[0123]The ink composition may be used as is, e.g. to print a clear coating
or layer or film on a substrate. Alternatively, the composition may be
used as a vehicle e.g. for carrying a dye in solution or a dispersed
pigment in known manner for printing of text or images. The term colorant
as used herein may refer to just a colorant, or it may refer to a
colorant in combination with, for example, a dispersant of some kind. The
colorants used herein are dyes or pigments, more preferably pigments. The
colorant may be any colour, but preferably the colorant is cyan, magenta,
yellow or black. The colouring agent generally comprises a colourant
which may be self-dispersed, polymer-dispersed or surfactant-dispersed.
When the colorant is self-dispersed the colorant is synonymous with the
colouring agent. Self-dispersed pigment refers to pigments that have been
chemically modified with a charge or a polymeric group, wherein the
chemical modification aids the pigment in becoming and/or substantially
remaining dispersed in a liquid vehicle. When the pigment is a
self-dispersing pigment the charging moiety is covalently-linked to the
pigment. Surfactant-dispersed pigment refers to pigments that utilize a
surfactant dispersant to aid the pigment in becoming and/or substantially
remaining dispersed in a liquid vehicle. The pigment dispersion should
contain enough dispersant to stabilize the pigment particle dispersion,
but not so much as to adversely affect properties of the dispersion such
as viscosity, stability, and optical density. The colorant may be chosen
from a wide range of conventional colorants, preferably pigments.
Preferably, the pigment is a white pigment, a black pigment, a blue
pigment, a brown pigment, a cyan pigment, a green pigment, a violet
pigment, a magenta pigment, a red pigment, or a yellow pigment, or shades
or combinations thereof. Suitable classes of coloured pigments include,
for example, anthraquinones, phthalocyanine blues, phthalocyanine greens,
diazos, monoazos, pyranthrones, perylenes, heterocyclic yellows,
quinacridones, diketopyrolopyroles, and (thio)indigoids. Such pigments
are commercially available in either powder or press cake form from a
number of sources including, BASF Corporation, Engelhard Corporation and
Sun Chemical Corporation. Examples of other suitable coloured pigments
are described in the Colour Index, 3rd edition (The Society of Dyers and
Colourists, 1982).

[0124]For the printing of black text or images it is appropriate to use
aqueous dispersions of oxidized carbon black pigment, such as the easily
dispersible NIPex and Special Black ranges of pigments (Degussa AG).
Special Black 250 (Degussa AG), which finds particular application in
radiation-cured printing inks, is preferred. (NIPex is a Trade Mark).

[0125]Preferably, in embodiments of the invention where pigments are used,
the pigment particle size is as small as possible to enable a stable
dispersion of the particles in the liquid vehicle and to prevent clogging
of the ink channels or nozzle when the ink is used in an inkjet printer.
Preferred particle average diameters are generally from about 0.001 to
about 0.5 microns, although the particle size can be outside this range
in specific embodiments. Preferably, at least 70% of the pigment
particles should have an average particle diameter of less than about 150
nm for carbon blacks and less than about 250 nm for colour pigments.

[0126]In embodiments including pigment, the composition preferably
includes at least one dispersant in small amount (say up to 1% by weight)
to improve dispersion of the pigment during milling. Suitable dispersants
include, but are not limited to the following:

[0127]Dispersants e.g. Solsperse 20,000 in an amount in the range 0.5 to
0.20% by weight, together with the synergist Solsperse 12,000 in an
amount in the range 0.01 to 0.10% have been found to give good results.

[0128]When dyes are used in the inkjet inks of the invention, any suitable
commercially available dye may be used to impart the desired colour
characteristics to the inkjet ink. Suitable anionic and cationic dyes are
well known for use in inkjet inks, and include, but are not limited to
the examples listed herein. Most inkjet ink dyes are anionic; however,
cationic dyes may also be used. Anionic dyes are those in which a
negative charge is localized on one atom or spread over the entire
molecule. Cationic dyes are those in which a positive charge is localized
on one atom or spread over the entire molecule.

[0131]In embodiments of the invention where dyes are used, the dye is
present in the inkjet ink composition in any effective amount to provide
a desired colour. Typically the dye is present in an amount of from about
1 to about 5% by weight of the ink composition, and preferably from about
1 to about 3% by weight (wherein the amount refers to an amount of dye
molecules present in the ink), although the amount can be outside this
range. A mixture of dyes in the proportions desired to obtain a specific
shade may also be employed. Similarly, in embodiments of the invention
where pigments are used, the pigment may be present in the inkjet ink
composition in any effective amount. Typically, the pigment is present in
an amount of from about 1% to about 10% by weight of the ink composition
and preferably from about 1% to about 5% by weight, although the amount
can be outside of this range. Where both dyes and pigments are
incorporated into the inkjet ink composition, the weight percentage of
the combined colorant may be adjusted accordingly.

[0132]It will also be appreciated by those skilled in the art that other
functional compositions suitable for inkjet printing are also possible
for use in the present invention in place of a colourant. Particulate
matter such as those suitable for use in preparing conductive regions
e.g. Cu, Ag, Au, and alloys of such, and those suitable for use in
preparing magnetic regions e.g. Fe and Co, and alloys of such, are also
possible for use in the present invention.

[0133]In addition, if necessary, additives such as pH adjusting agents,
rust preventives, fungicides, antioxidants, evaporation accelerators,
chelating agents, and water-soluble polymers other than the above
described components, may be added into the inks used in the present
invention.

[0134]Further details of conventional and optional ingredients for
printing inks and their use are given, e.g. in U.S. Pat. No. 6,294,592.

[0135]Compositions in accordance with the invention can have minimal
fluctuation of viscosity, excellent stability of components and can be
free from deterioration of dispersibility (for dye-based inks).

[0136]The compositions of the invention are used in conventional manner,
using a thermal inkjet printer. The compositions are printed onto the
intended substrate and exposed to an appropriate curing regime depending
on the nature of the curable materials, e.g. involving exposure to UV
light, heat, etc.

[0138]Even on difficult to handle non-porous substrates, the invention can
provide compositions that are fast drying and produce good quality prints
of high durability with good adhesion to the substrate. In particular,
the resulting prints may satisfy the requirements of good scratch
resistance, water-fastness, flexibility and optical density (for coloured
inks). The compositions find particular application in industrial
printing onto non-porous substrates and semi-porous substrates.

[0139]It is seen then that there is a particular need for a thermal inkjet
ink capable of printing on non-porous substrates to produce images with
good adhesion to these surfaces that is resistant to moisture and wet
rub. It would further be desirable to develop an ink with high moisture
tolerance that does not cause corrosion of the printhead. This need is
met by the ink composition according to the present invention, wherein
enhancement in permanence and adhesion of the ink to non-porous
substrates is achieved.

[0140]According to a preferred aspect of the present invention, there is
provided a pigment dispersion comprising: at least one pigment having an
average particle diameter of 150 nm or less; and a dispersion medium
containing as major components at least three radiation-curable
oligomers, at least one curable monomer, and at least one dispersant and
one synergist, having a combined viscosity of less than 30 mPas at a
temperature of 25° C.; wherein the mixing ratio of the pigment is
confined within the range of 1 to 5% by weight based on a total weight of
the ink composition.

[0141]According to another preferred aspect of the present invention,
there is provided a single-phase radiation-curable liquid ink precursor
having a viscosity of less than 11 mPas, more preferably less than 6.5
mPas at room temperature (25° C.) and suitable for thermal inkjet
printing, comprising a pigment dispersion which comprises at least one
pigment having an average particle diameter of 150 nm or less; and a
dispersion medium containing as major components at least three
radiation-curable oligomers, at least one curable monomer, and at least
one dispersant and one synergist, having a combined viscosity of less
than 30 mPas at a temperature of 25° C., and further containing
water, at least one free-radical initiator, at least one or more curable
monomers, and one or more co-solvents for the curable materials, and
optionally containing one or more surfactants; wherein the mixing ratio
of the pigment is confined within the range of 1 to 5% by weight based on
a total weight of the ink composition.

[0142]In a further aspect, the invention provides a single phase aqueous
curable composition suitable for thermal inkjet printing, comprising
water in an amount not exceeding 35% by weight of the weight of the
composition, and one or more curable materials. Water is preferably
present in an amount not exceeding 30% by weight. Water is preferably
present in an amount of at least 10% by weight.

[0143]The invention will be further described, by way of illustration, in
the following Examples.

EXAMPLES

[0144]In the Examples all the quantities are % by weight and all
viscosities are at 25° C. unless otherwise specified.

[0145]Viscosity measurements were performed using a Brookfield DV-II+
viscometer operating with a rotational speed of 60 rpm at a temperature
of 25° C. Briefly, 17.5 ml of ink was transferred to the chamber,
to which a suitable spindle was then lowered into the chamber and left
until the temperature stabilized. Measurements were taken every 30, 60,
120 and 300 seconds, until a reproducible viscosity reading could be
obtained. [Units: 1 mPas 1 cP]

[0146]The following radiation-curable ink formulations were made from the
listed ingredients given in Tables 1, 2 and 3 below. All of the inks in
accordance with the invention are in the form of single-phase aqueous
radiation-curable compositions, formulated either as a clear ink or as an
ink carrying a dispersed pigment or a dissolved dye.

[0147]Inks NJ018, NJα and NJβ are comparative examples
indicated by "Comp", having unsuitable quantities of water. NJ018 has
insufficient water, and caused overheating of the inkjet cartridge.
NJα and NJβ have too much water and had unacceptable wet rub
resistance properties.

[0148]The various performance ratings given in Tables 1 and 2 are rated
subjectively on a scale of 1 to 5, with 5 being best, with scratch
resistance and wet rub resistance being for prints on white Melinex
polyester substrate (Melinex is a Trade Mark), a non-porous material.

[0149]In Table 3, wet rub resistance values are the number of rubs of a
print on Melinex with a wet lint-free wipe, with moderate pressure
applied, before any removal was observed.

[0150]The exemplified inks were prepared by mixing the liquid curable
materials and photoinitiators, to produce a UV-curable mixture (referred
to as part A). Solid materials, particularly n-vinyl acetamide and
n-methyl-n-vinyl acetamide, were dissolved in water and co-solvent to
produce a second mixture (referred to as part B). Parts A and B were
mixed together along with remaining ingredients, namely surfactant,
wetting agents etc. and with dye or pigment dispersion, if present.

[0151]The inkjet inks of the present invention can be prepared by any
process suitable for preparing aqueous-based inks. The pigmented ink is
prepared by premixing the selected pigment(s) and dispersant in water. In
the case of dyes, some of the same factors apply except that there is no
dispersant present and no need for pigment deaggregation. The dye-based
ink is prepared in a well-agitated vessel rather than in dispersing
equipment. Co-solvents may be present during the dispersion. The
dispersing step may be accomplished in a horizontal mini mill, a ball
mill, or an attritor. It is generally desirable to make the pigmented
inkjet ink in concentrated form. The concentrated pigmented inkjet ink is
subsequently diluted to the appropriate concentration for use in the
inkjet printing system. This technique permits preparation of a greater
quantity of pigmented ink from the equipment. If the pigment dispersion
is made in a solvent, it is diluted with water and optionally other
solvents to obtain the appropriate concentration. If the pigment
dispersion is made in water, it is diluted with either additional
deionised water or water-soluble solvents to make a pigment dispersion of
the desired concentration. By dilution, the ink is adjusted to the
desired viscosity, colour, hue, saturation density, and print area
coverage for the particular application.

[0152]Pigmented Thermal Inkjet Inks

[0153]In a preferred method, the pigmented inks were prepared according to
the following procedure.

[0154]Pigment Dispersion

[0155]A typical mill base (i.e. dispersion medium) was prepared by mixing
the following radiation-curable materials: SR 508 (26.7%), SR 454
(13.3%), SR 285 (26.7%) and n-acryloyl morpholine (33.3%) in a high shear
mixing apparatus. The mill base mixture was used to produce a pigment
dispersion comprising 77.6% of the mixture, 1.6% Solsperse 20,000
dispersant, 0.8% Solsperse synergist and 20% Special Black 250 pigment.
The dispersant and synergist were added to the mill base mixture, and the
combined solution high shear mixed for 10 minutes. Special Black 250
pigment (Degussa AG) was then added to the solution and the entire
mixture was high shear mixed for a further 10 minutes. The
solvent-dispersed pigment was then milled using an Eiger bead mill
operating in recirculating mode at 25° C. at a rate of 4000 rpm
for 4 hours (Eiger is a Trade Mark). The milling media used was 0.1 to
1.0 μm ceramic beads. The pigment particle size attained was <150
nm.

[0156]Liquid Inkjet Ink Precursor

[0157]Pigmented inkjet inks were prepared by adding the various components
in Tables 1 and 2 to the pigment dispersion according to the
compositions.

[0160]In a preferred method, the dye-based inks were prepared according to
the following procedure.

[0161]A typical dye-based thermal inkjet ink was prepared by mixing the
following radiation-curable materials: SR 508 (26.7%), SR 454 (13.3%), SR
285 (26.7%) and n-acryloyl morpholine (33.3%) in a high shear mixing
apparatus. To a 32.4% portion of this mixture was added Irgacure 2959
(4.41%), Darocur TPO (4.01%), NAM (2.66%), EGMBE (6.85%), water (20.88%)
and NVAM (4.27%), and the combined solution high shear mixed for a
further 10 minutes.

[0162]Orasol Black RLI (1.77%, Ciba-Geigy) was then added to the solution
and the entire mixture was high shear mixed for a final 10 minutes at
room temperature (25° C.). The resulting ink was then filtered
(Whatman GF/B glass microfibre filter, 1 μm) to give a final ink
suitable for thermal inkjet printing.

[0163]The compositions were printed onto a range of different substrates
at 600×600 dpi from a Wolke (Wolke is a Trade Mark) printer, or
from one of the following Hewlett Packard desktop printers: HP 6127 and
HP 850, using an HP45A cartridge (with capacity 43 ml ink). The printed
material was cured by exposure to UV from a 500 W Fusion Light Hammer 6
system with an `H` bulb, more preferably a `D` bulb, being conveyed below
the UV system at a line speed of 30 m/min. (Fusion Light Hammer 6 is a
Trade Mark). There was no need for a pre-curing drying step.

[0164]Preferred compositions include HM79 and NJ000. As well as having
excellent jetting reliability and wet-fastness, they also shows good
start-stop performance (i.e. where a cartridge can be left idle after a
period of continuous printing and on re-starting printing is readily
recoverable). No significant material was deposited onto the printhead
heaters after printing four full cartridges of these compositions.

[0165]The ink compositions in accordance with the invention were tested on
a wide range of substrates of non-porous, semi-porous and porous
materials, and were found to give good quality prints of high durability
with good adhesion to the substrate. The prints had good scratch
resistance, good resistance to water and other solvents, good flexibility
and good optical density (for coloured inks).

[0166]Performance data for composition HM79 on a range of different
non-porous substrates is summarised below, printed using a fusion
conveyor system Light Hamer 6 (H bulb) at a speed of 30 m/min. Results
are summarised in the table below.